An electric rotary machine can be an electric motor or a generator.
Electric motor is widely used in both industrial and consumer products such as pumps, compressors and fans. It is important to identify any potential faults in an electric motor and maintain it in good health. The failure modes of an electric motor include stator turn-turn short, broken stator windings, stator-frame insulation breakdown, deteriorated rotor eccentricity, broken rotor bars, rotor bar-bar short, shifted shaft, and bearing defects.
Fault detection techniques of a rotary machine mainly fall into two categories, online test and offline test. Online test is to measure dynamics of a rotary machine while the rotary machine is in operation. Online test typically requires various types of sensors and the output is also vulnerable to noise. Offline test is to measure statics of a rotary machine and it is more immune to noise. Offline tests can be low voltage, medium voltage and high voltage.
Frequency response is a digital signal processing (DSP) technique to uniquely identify a system. It is like a system's fingerprint and is a powerful tool to identify abnormality in a system. IEC 60076-18 standardizes the Sweep Frequency Response Analysis (SFRA) technique to measure frequency response of power transformers to detect faults. While there are similarities between power transformers and rotary machines, there is no standard for rotary machine testing based on frequency response. Additionally, IEC 60076.-18 does not describe how to properly interpret the test data from SFRA.
The difficulty of applying frequency response method in rotary machine fault detection lies in the fact that the frequency response is dependent on the rotor position. There have been attempts to apply the SERA technique on rotary machines: L. Lamarre and P. Picher: “Impedance characterization of hydro generator stator windings and preliminary results of FRA analysis”, Proc. Conf. Record 2008, IEEE Int. Syrup. Electr. Insul., pp. 227-230 (publication [1]); Martin Brandt, Slavomía Kascak: “Failure Identification of Induction Motor using SFRA Method”, ELEKTRO 2016, pp 269-272 (publication. [2]). However, either the rotor has to be removed or the rotor has to be set at a known position. Further, bulky and expensive equipment has to be used. This has complicated the test and analysis, and made it difficult to implement. Thus, there remains a considerable need for methods and apparatus that can reliably and conveniently test rotary machines.